Secondary batteries, for example, lithium metal batteries, lithium ion batteries, lithium polymer batteries, and so forth, that have an alkali metal, such as lithium metal, or an alloy or compound thereof in the negative electrode active material, have high capacities and as a result have been in the spotlight in recent years. A variety of materials have been investigated as rare metal-free positive electrode active materials for such secondary batteries, with a view to boosting their performance and capacity while lowering their cost. For example, Patent document 1 describes a positive electrode active material based on an olivine-type transition metal phosphate complex represented by the general formula AyMPO4 (in the formula, A is an alkali metal, M is a transition metal comprising the combination of both Co and Fe, and 0<y<2). Among transition metal phosphate complexes, lithium manganese phosphate (LiMnPO4), in which the alkali metal is Li and the transition metal is Mn, are known to have a wider atomic spacing between metal elements in the crystal structure than positive electrode active materials based on other transition metal oxides and even among olivine-type transition metal phosphate complexes are known in particular to have poor rate characteristics. LiMnPO4 has a theoretical capacity of approximately 170 mAh/g, or about the same as LiFePO4, but numerous reports have indicated that its utilization ratio is much worse than that of LiFePO4 even under low rate discharge conditions (for example, Non-Patent document 1). For example, in the case of LiFePO4, there have been efforts to improve its rate characteristics through the use of a carbon coating (Non-Patent document 2), a noble metal support (Non-Patent document 3), an increase in the reaction surface area by low-temperature synthesis microfine-sizing (Non-Patent document 4), and so forth, and improvements in the rate characteristics have in fact been recognized. In the case of LiMnPO4, however, there have been no reports of a method for which a clear improvement in the rate characteristics has been seen.
Patent document 1: Japanese Patent No. 3,523,397
Non-Patent document 1: A. K. Padhi, K. S. Nanjundaswamy and J. B. Goodenough, J. Electrochem. Soc., Vol. 144, No. 4, 1188-1193 (1997)
Non-Patent document 2: Z. Chen and J. R. Dahn, J. Electrochem. Soc., Vol. 149, No. 9, A1184-A1189 (2002)
Non-Patent document 3: K. S. Park, J. T. Son, H. T. Chung, S. J. Kim, C. H. Lee, K. T. Kang and H. G. Kim, Solid State Comm., Vol. 129, 311-314 (2004)
Non-Patent document 4: A. Yamada, S. C. Chung and K. Hinokuma, J. Electrochem. Soc., Vol. 148, No. 3, A224-A229 (2001)